Friday, May 20, 2011

This is a reconstruction of X. sapingensis, based on the fossil. (Credit: Sterling Nesbitt)

ScienceDaily (May 19, 2011) —
A fossil unearthed in China in the 1970s of a creature that died about
247 million years ago, originally thought to be a distant relative of
both birds and crocodiles, turns out to have come from the crocodile
family tree after it had already split from the bird family tree,
according to research led by a University of Washington paleontologist.

The only known specimen of Xilousuchus sapingensis
has been reexamined and is now classified as an archosaur. Archosaurs,
characterized by skulls with long, narrow snouts and teeth set in
sockets, include dinosaurs as well as crocodiles and birds.

The new examination dates the X. sapingensis specimen to the
early Triassic period, 247 million to 252 million years ago, said
Sterling Nesbitt, a UW postdoctoral researcher in biology. That means
the creature lived just a short geological time after the largest mass
extinction in Earth's history, 252 million years ago at the end of the
Permian period, when as much as 95 percent of marine life and 70 percent
of land creatures perished. The evidence, he said, places X. sapingensis on the crocodile side of the archosaur family tree.

"We're marching closer and closer to the Permian-Triassic boundary
with the origin of archosaurs," Nesbitt said. "And today the archosaurs
are still the dominant land vertebrate, when you look at the diversity
of birds."

The work could sharpen debate among paleontologists about whether
archosaurs existed before the Permian period and survived the extinction
event, or if only archosaur precursors were on the scene before the end
of the Permian.

"Archosaurs might have survived the extinction or they might have
been a product of the recovery from the extinction," Nesbitt said.

The research is published May 17 online in Earth and Environmental
Science Transactions of the Royal Society of Edinburgh, a journal of
Cambridge University in the United Kingdom.
Co-authors are Jun Liu of the American Museum of Natural History in
New York and Chun Li of the Institute of Vertebrate Paleontology and
Paleoanthropology in Beijing, China. Nesbitt did most of his work on the
project while a postdoctoral researcher at the University of Texas at
Austin.

The X. sapingensis specimen -- a skull and 10 vertebrae --
was found in the Heshanggou Formation in northern China, an area with
deposits that date from the early and mid-Triassic period, from 252
million to 230 million years ago, and further back, before the mass
extinction.

The fossil was originally classified as an archosauriform, a "cousin"
of archosaurs, rather than a true archosaur, but that was before the
discovery of more complete early archosaur specimens from other parts of
the Triassic period. The researchers examined bones from the specimen
in detail, comparing them to those from the closest relatives of
archosaurs, and discovered that X. sapingensis differed from virtually every archosauriform.

Among their findings was that bones at the tip of the jaw that bear
the teeth likely were not downturned as much as originally thought when
the specimen was first described in the 1980s. They also found that
neural spines of the neck formed the forward part of a sail similar to
that found on another ancient archosaur called Arizonasaurus, a very
close relative of Xilousuchus found in Arizona.

The family trees of birds and crocodiles meet somewhere in the early
Triassic and archosauriforms are the closest cousin to those archosaurs,
Nesbitt said. But the new research places X. sapingensis
firmly within the archosaur family tree, providing evidence that the
early members of the crocodile and bird family trees evolved earlier
than previously thought.
"This animal is closer to a crocodile, but it's not a crocodile. If
you saw it today you wouldn't think it was a crocodile, especially not
with a sail on its back," he said.

The research was funded by the National Science Foundation, the
Society of Vertebrate Paleontology, the American Museum of Natural
History and the Chinese Academy of Sciences.

Story Source:

The above story is reprinted (with editorial adaptations by ScienceDaily staff) from materials provided by University of Washington. The original article was written by Vince Stricherz.

My interest in this paper is that it discusses the ancestry of birds, which evolved from dinosaurs; also, that the ancestor of all birds evolved much earlier than believed. Re: the discussion of bats and thecodonts, that is something you need to take to the authors of said paper.